It is well known in the art that the properties of lubricating oils for internal combustion engines can be improved by the addition of molybdenum-containing materials having anti-wear and/or friction-reducing properties. Anti-wear materials prevent or slow down the wear that occurs between metal surfaces which are in close rubbing contact, particularly in the boundary lubrication regime where metal to metal contact occurs. They are known to work by forming a permanent film on the surface metal which protects it from wear phenomena which would otherwise lead to removal of metal, loss of performance and eventual welding together of metal parts. Friction-reducing materials act by reducing the friction between metal surfaces when they rub together, resulting in improved fuel economy. They are known to give benefit in both the hydrodynamic and boundary lubrication regimes.
These molybdenum-containing materials typically contain a source of active sulphur. In Column 1, line 53 to Column 2, line 2 of US-A-4,692,256 it is stated: "It has been considered essential that organic molybdenum compounds useful as lubricant additives should contain sulfur atoms in the molecules of the compounds. That is, it has been considered that the lubricating performance can be obtained by the formation of molybdenum disulfide on the lubricating surface by molybdenum and sulfur contained in the molecules. However, the present inventors have assumed that active sulfur atoms contained in the molecules may have undesirable effects in view of the metal corrosion and have made an earnest study in order to overcome the contraction. As a result, it has surprisingly been found that although the product obtained by the reaction between a molybdenum compound and an amino compound has no substantial performance when used alone as a lubricant additive, it exhibits extremely satisfactory lubricating performance when combined with a sulfur-containing compound". Examples of such sulphur-containing compounds are said in Column 4, lines 11 to 19 to include sulphurised fatty acids, sulphurised oils and fats, sulphurised olefins, disulphide compound such as dibenzyl sulphide, dithiocarbamate such as butylphenyl thiocarbamate disulphide, phosphorous- and sulphur-containing compounds such as tetraalkylthioperoxy phosphate, molybdenum dithiocarbamate, molybdenum dithiophosphate and zinc dithiophosphate.
US-A-4,266,945 discloses, as extreme-pressure and friction modifying additives in lubricants and fuels, molybdenum-containing compositions substantially free of Group IA and IIA metals which are prepared by reacting, at a temperature up to about 200.degree. C., a mixture comprising (A) at least one acid of molybdenum, or salt thereof; (B) at least one phenol, or condensation product of said phenol and at least one lower aidehyde; and (C) at least one compound selected from the group consisting of (1) amines having the formula R.sup.1 (R.sup.2)NH wherein R.sup.1 is an aliphatic hydrocarbon-based radical and R.sup.2 is hydrogen or an aliphatic hydrocarbon-based radical; (2) condensation products of said amines with at least one lower aidehyde; and (3) salts of (1) or (2). The molybdenum-containing compositions may be used in conjunction with at least one compound containing active sulphur, e.g. a sulphurised olefin, a sulphurised mercaptan, a sulphurised phenol, or a dialkyl xanthate or carbamate.
The reagent (B) can be a phenol. The term "phenol" is defined as a compound containing a hydroxyl group bound directly to an aromatic ring and is said to include compounds having more than one hydroxyl group bound to an aromatic ring, and also alkylalkenyl phenols. No other phenols are mentioned. Preferred are phenols containing at least one alkyl substituent containing about 3 to 100 and especially about 6 to 20 carbon atoms, with monoalkylphenols being particularly preferred.
A reference to US-A-4,266,945 can be found in Column 1, lines 30 to 34 of US-A-4,466,901 which discloses a lubricating oil anti-friction additive composition prepared by reacting a phenolic compound, with a molybdenum compound, an amine compound and sulphur or a sulphur-yielding compound.
In Column 1, lines 52 to 56 of US-A-4,466,901, it is stated that molybdenum compounds produced by prior art methods including that of US-A-4,266,945 potentially suffer from either economic inefficiencies or from changing product requirements, i.e. they do not meet current environmental standards. Furthermore, in Column 2, lines 4 to 10, it is stated that whilst these molybdenum compounds can improve the characteristics of lubricating oils, they suffer the additional drawbacks in that they are often uneconomical or difficult to prepare, cannot be prepared in a batch process, and may or may not have sufficient amounts of sulphur incorporated within the additive to benefit fully from the molybdenum contained therein.
SU-A-1143767 discloses a lubricating composition containing (a) 1.5 to 1.53% w 2-mercaptobenzothiazole (anti-wear and anti-scuff additive), (b) 0.201 to 0.204% w p-hydroxyphenylene diamine (antioxidant additive), (c) 0.5 to 2% w molybdenyl chloride of the formula MoO.sub.2 Cl.sub.2.HL where HL is salicylaldehyde/2,4,6-trimethyl- aniline or salicylaldehyde/tert-butylamine (each of these being a Schiff base and therefore containing the group CH.dbd.N--), and (d) the balance being a synthetic oil based on an ester of pentaerythritol and C.sub.5 -C.sub.9 aliphatic acids. Test data in the Table in Columns 5 and 6 of SU-A-1143767 (particularly the results for Composition 2 compared with those for Compositions 3 to 6) show that the anti-wear and anti-scuff properties of a lubricating composition containing components (a), (b) and (d) were enhanced by the addition of the molybdenyl chloride (c). There is no test data on any lubricating compositions containing components (c) and (d) only and therefore there is no suggestion of any anti-wear or friction-reducing capabilities of the molybdenyl chloride (c) per se.
SU-A-1054406 discloses a lubricating composition with improved lubricity containing, as anti-wear additive, 0.05 to 0.25% w molybdenyl chloride bis(salicylaldehyde anilinate), 0.45 to 2.25% w thioglycolic acid, and the balance being a synthetic oil based on an ester of pentaerythritol and C.sub.5 -C.sub.9 aliphatic acids. The composition is referred to in SU-A-143767 discussed above where it is mentioned that the use of molybdenyl chloride bis(salicylaldehyde anilinate) is problematic since it can only be incorporated into the synthetic oil after it has first been dissolved in thioglycolic acid, which leads to the formation of sediment in the oil during long-term operation.
It has now surprisingly been found possible to prepare molybdenum-containing complexes from carboxylic compounds, being free of active sulphur, which show advantageous friction-reducing properties.